The present invention relates to coatings that protect and insulate surfaces from high temperatures.
At the present time TBC""s are used to protect metal components exposed to high temperatures by reducing the temperature of the metal. For example, in gas turbines they are used on combustion hardware, after burners, vanes, blades, fuel nozzles and spray bars. More particularly, these fuel nozzles operate in an envelope of 700xc2x0 F. to 1300xc2x0 F. air. The fuel nozzles will become clogged if the temperature of the fuel becomes greater than about 400xc2x0 F. because the fuel will form deposits at these high temperatures. The fuel is, of course, used to provide the energy to the turbine section of the gas turbine engine in order to produce useful thrust, work and/or heat. The primary cause of the fuel becoming too hot is from the heat of the air surrounding the fuel nozzle. The purpose of the TBC is to reduce the amount of heat flowing from this air into the fuel nozzle, and thence, into the fuel.
The difficulties encountered in making a suitable TBC are based on the fact that such protective coating must have a large number of mechanical and thermal properties. The TBC""s must be capable of adhering to the metal and capable of being adhered thereto by a simple and low cost method. Also, the TBC""s must protect the metal against heat, corrosion, and environmental damage.
TBC""s currently in use for fuel nozzles are made by a plasma spray process. Those used for blades and vanes may also be made by an electron-beam, physical deposition process, but this is a very costly process. The plasma spray processes are inherently chaotic in nature, can be difficult to control and can produce unacceptable coatings while still using nominally the same materials and processing conditions. Such unacceptable coatings are those that readily suffer vibrational and/or thermal cycling damage, detachment from the metal surface and subsequent obstruction by the debris of critical air passages in the fuel nozzles. Plasma spray processes are also relatively expensive and require specialized set-up and long processing times.
The present invention provides a composition and a method for coating surfaces. The method comprises contacting a surface comprising metal with the coating composition. The coating composition has a polymerizable hydrated silicate powder comprising alkali metal ions, a forming agent comprising polyvalent metal ions, and an amount and form of moisture effective to promote ion exchange between the polyvalent metal ions and the alkali metal ions. The coating composition is cured under first curing conditions and for a time effective to 1) initiate polymerization of the silicate powder, and 2) to promote the ion exchange, producing an intermediate coating composition comprising bound water molecules. The first curing conditions are effective to maintain a sufficient amount of the moisture in the coating composition to promote ion exchange while under the first curing conditions. Sufficient energy is imparted to the coating composition at a rate and under second curing conditions effective to drive the bound water molecules from the intermediate coating composition and to produce a durable, adhesive protective coating.
The instant coating composition adheres to metal surfaces and has excellent mechanical, thermal barrier, and environmental protective properties. These properties are achieved using a combination of a polymerizable silicate composition and a forming agent cured under conditions effective to remove substantially all of the water from the resulting coating.
The coating composition is formed using a polymerizable spray-dried hydrated alkali metal silicate powder. Any polymerizable spray-dried hydrated alkali silicate powder may be used in the present invention. Briefly, the powder is prepared by conventionally spray drying a solution of alkali metal silicate under conditions effective to produce a free-flowing powder. One example of a suitable polymerizable spray-dried hydrated alkali silicate powder can be found in U.S. Pat. No. 4,030,939. Sodium silicate powder is preferred due to its wide availability and low cost. The spray-dried hydrated sodium silicate powder preferably has a ratio of SiO2/Na2O of from about 2 to 1 to about 3.5 to 1, most preferably 2.4:1. Such powders are commercially available under the trademark BRITESIL sold by Philadelphia Quartz Corp.
In order to form the coating, a forming agent capable of forming weak acids is combined with the alkali metal silicate powder. Suitable forming agents include but are not necessarily limited polyvalent metal ions, such as zinc, aluminum, and zirconium, and mixtures thereof Preferably, the forming agent is in a finely ground state, from about minus 200 mesh (Tyler Standard) to about minus 400 mesh, most preferably about 5xcexc or less. The smallest available particle size is desirable because it minimizes the reaction time and increases the rate at which alkali silicate is converted to polysilicic acid. In a preferred embodiment, from about 5 to about 20 parts by weight forming agent is added.
For optimum strength and resistance against shock, it is preferred to add certain siliceous fillers which can also react in forming the preferred protective coating. When the coating is heated to greater than 1,000xc2x0 F., the fillers will mineralize and hybridize to the corresponding silicate. Such materials include siliceous sand, silica flour, fly ash clays, other argillaceous materials of high silicate content including, rice hull, diatomaceous silica, volcanic ash or mixtures thereof. Of these, silica flour is preferred due to its high availability and low price. For optimum reaction, finely ground filler materials, such as minus 200 mesh (Tyler Standard) should be used.
The coating composition is mixed with water and it has been found that for optimum strength, integrity and continuity of the coatings; namely, to prevent shrinking, cracks and the like, it is preferred to have a ratio of water to spray-dried hydrated sodium silicate powder of from about 0.9:1 to about 1.1:1. Limiting the amount of water is important to provide low porosity, final strength integrity and the desired application properties, such as trowelable, injectable, and castable rheologies. The composition can vary from Newtonian to thixotropic slurries depending on the application method to be used; i.e., dip coating, injection into fine capillaries and annuli, spray coating, or other conventional coating techniques.
Without being bound to any particular theory, it is believed that a polymerization reaction takes place as the forming agent hydrolyzes in the presence of water to liberate small quantities of ionic metal oxides and/or hydroxides. (e.g. Zn++, (OH)2xe2x88x92). The liberated ions induce a steady polymerization of silicic acid hydrogel which is liberated by the neutralization of sodium silicate by, for example, by ion exchange with the polyvalent metal ions. As the ionized metal is consumed it is converted into a silicate polymer of high molecular weight products that solidifies around the filler, binding the material together. The siliceous and uniquely charged colloidal silica hydrogel attacks metal and mineral surfaces to provide the basis for forming a silicate bond to that surface. One example of a divalent metal/SiO2 polymerization is illustrated by the following formula:
[xe2x80x94(SiO2)xe2x80x94Oxe2x80x94Znxe2x80x94Oxe2x80x94(SiO2)xe2x80x94Oxe2x80x94Znxe2x80x94Oxe2x80x94(SiO2)xe2x80x94Oxe2x80x94]n
where n is from about 1,000 to about 1,000,000, preferably from about 10,000 to about 1,000,000.
In a preferred embodiment, lightweight ceramic microspheres having insulative properties are used in the coating composition. The microspheres provide heat resistance and reduce the overall density of the resulting coating. Suitable microspheres are made in various diameter size and are commercially available from companies such as Philadelphia Quartz Corp., 3M Corp. and the like. The preferred microspheres are hollow ceramic spheres and can be utilized in various micron sizes, where hollow spheres having a size of 5-200 xcexcm are most suitable and economical. In a preferred embodiment, said microspheres comprise from about 10 to about 48 parts by weight of said coating composition.
Preferably heat resistant whiskers and/or fibers are added to the composition to provide resistance to shrinkage and thermal stressing. The length of the whiskers and/or fibers can vary, but ordinarily the whisker sizes may range from 5-100 xcexcm in length and the fibers range from 300-3550 xcexcm in length.
A wide range of proportions of materials can be utilized and the table that follows sets forth an operative range and the preferred range of materials.
To form the coating composition, the components noted above are thoroughly admixed, vibrated and evacuated to ensure that all entrapped air is removed, and the resulting aqueous slurry is applied to a metal surface to the desired thickness or injected into cavities and/or annuli. The coating composition when formed may be somewhat thixotropic and can be built up to various thicknesses or reduced to a Newtonian fluid. Bentonite may be used to adjust the thickness of the composition depending upon the desired application properties. The optimum thickness of any particular coating is readily calculable by routine experimentation and is based mainly on the temperature to which the metal is to be exposed with higher temperatures requiring thicker coatings. Ordinarily, it has been found that suitable protective coatings can be formed with temperatures up to 2,000xc2x0 F. by having a coating thickness of 0.5-6 mm. As noted, the optimum thickness for any particular metal composition can be determined by routine experimentation by simply coating the composition to various thicknesses and noting the properties of the coating after being exposed to the temperature conditions.
The resulting coating is a silica polymer matrix comprising silica, polysilicic acid, Si-polyvalent metal ion complexes and high ratios of NaO(SiO2)n where n is greater than about 2.4, preferably from about 100 to about 1000. The liberated sodium is mineralized by the silica fillers and fly ash silicates. The silica, and Si-polyvalent metal ion complexes have a particle size effective to produce coatings having a thickness of 10 mm or less, depending upon the end use for the article being coated. The polysilicic acid and Si-polyvalent metal ion complexes are substantially free of bound water molecules. The insulative ceramic microspheres are uniformly embedded in the matrix. Substantially free of bound water molecules is used herein to mean a coating that will not delaminate, liquefy, or flake when subjected to heat treatments that surround the coated article at temperatures above about 1,000xc2x0 F.
In a preferred embodiment, the coating is cured slowly with increasing heat to produce chemically bonded and cured coatings that are highly adherent to metals and resistant to delamination, cracking and flaking during use, when exposed to rapid heat at temperatures in excess of 2,000xc2x0 F. More particularly, the coating is initially cured in a closed environment overnight, preferably from about 8 to about 24 hours, more preferably from about 8 to about 20 hours, and most preferably from about 8 to about 10 hours, at ambient temperature (ie. about 70xc2x0 F. to about 80xc2x0 F). xe2x80x9cClosed environmentxe2x80x9d is used herein to mean an environment that is sealed from air flow such that substantially all of the air inside the container is not allowed to mix with air outside the container and a water content of at least 15% is maintained in the environment to facilitate the curing reaction. Likewise, open environment means that the air in the container is allowed to mix with air outside the container. The coating then is cured at a temperature of from about 150xc2x0 F. to about 250xc2x0 F. for about 3 hours at a pressure of from about 4 psi to about 135 psi. The stoichiometric ratio of the polyvalent metal ion to the alkali metal silicate is at least 1 to assure complete ion exchange between the polyvalent metal ions and the silicate to convert the mixture to an insoluble silicate polymer.
Without being bound by any particular theory, applicant believes the initial ambient temperature cure combined with the subsequent relatively mild heat curing step facilitates the ion exchange between the polyvalent metals (e.g. Zn, Zr, Al) and the alkali metal silicate powder to form the silicate polymer or ceramic product. Preferably, the coated articles are heated in an environment that provides uniform heat surrounding the coated article. A closed environment is preferred so that the water present in the coating composition is available to promote the ionization of the polyvalent metals such that they then react with the alkali metal silicate molecules to polymerize the composition. Complete or near complete ion exchange is important in order to assure the absence in the cured coating of un-reacted, water soluble akali metal silicates in the coating. If un-reacted water soluble alkali metal silicates are not bound during the polymerization they could cause the coating to melt, liquefy, flux and fail when exposed to temperatures in excess of 1,000xc2x0 F.
The coating is subsequently heated at increasing temperatures starting at about 250xc2x0 F. and increasing at a rate of about 20xc2x0 F. or less per minute to about 1,000xc2x0 F. or more in an open environment at atmospheric pressure. The gradual increase in temperature is believed to effectively remove water bound to the polyvalent metal (i.e.
Sixe2x80x94OH) at a rate that avoids disruption of the coating. Once the temperature reaches 1,000xc2x0 F., the article is allowed to cool to ambient temperature. The gradual increase in temperature removes bound water from the coating so that the bound water is not available to distill upon rapid heating and cause the coating to crack or delaminate.